The effects of 3-methyl-4-(5′-nitrofurfurylidene-amino)-tetrahydro-4H-1,4-thiazine-1,1-dioxide [Lampit, Bayer 2502] on the intracellular cycle of Trypanosoma cruzi were studied with an in vitro steady-state culture system that permitted the continuous analysis of individual host cell-parasite interactions. Lampit concentrations of 10-4 and 10-5 M significantly affected the ability of trypomastigotes to penetrate vertebrate cells. Lampit concentrations above 10-4 M were toxic to the host cell population and Lampit concentrations below 10-5 M did not affect the ability of trypomastigotes to penetrate vertebrate cells. Lampit concentrations of 10-5 M or greater completely inhibited the intracellular cycle of T. cruzi whereas Lampit concentrations of 10-7 M or less had no effect on the intracellular cycle. Continuous perfusion with 10-6 M Lampit resulted in a linear increase in intracellular parasite reproduction time. A stable strain of T. cruzi was produced in vitro that was resistant to a 100-fold greater concentration of Lampit than the parent strain.